The YANMAR 3YM20 is a robust Marine Diesel Engine designed for reliable performance in maritime applications. This naturally aspirated engine features a displacement of approximately 1.266 liters, delivering a typical power output of 20 HP (14.7 kW) at 3200 RPM. Its engineering prioritizes durability and efficiency, making it a dependable choice for various marine vessels. The engine utilizes a forced lubrication system and is compatible with both raw water and freshwater cooling methods, ensuring optimal operation under diverse conditions. This Service and Repair Manual provides comprehensive guidance for the YANMAR 3YM20 Marine Diesel Engine. It covers essential maintenance procedures, detailed repair instructions, and step-by-step disassembly and reassembly processes. Critical technical specifications, including bore, stroke, and fuel system details, are included to aid in accurate diagnostics and servicing. This manual is intended for professional technicians and DIY mechanics with basic electrical and mechanical knowledge, empowering them to effectively maintain and repair the engine.